My vote goes to the 20ga also. Definetly cheaper to feed shells through it. 20ga shells are about 1/2 the price of .410 shells. You can also find 20ga shells anywhere and a huge variety of them as well. THe same can't be said for the .410.

However, I suggest that dad throws in a little cash get him a youth sized pump. I like single shot rifles but I think he would be much happier with a pump should he need a follow up shot.